A defibrillator is a medical device used to discharge a substantial amount of energy into cardiac tissue. The defibrillator is used to arrest a potentially life-threatening tachyarrhythmia. Defibrillators take many different forms, for example, one type is a unit where all components are external to the patient. The components are portable allowing it to be brought to the patient. Moreover, it can be used on a number of different patients. Defibrillators are also implantable devices placed subcutaneously in the patient as a dedicated system for the patient. An implantable defibrillator has the benefit of detecting a tachyarrhythmia and immediately providing a therapy that arrests the tachyarrhythmia.
In general, a high energy pulse or pulses are applied to the targeted tissue requiring treatment. One common method to generate a high energy pulse is to convert electrical energy from a low voltage/low current source to a high voltage. One approach to achieve this is known as a DC-DC converter. The DC-DC converter utilizes a capacitor and a transformer to generate the high voltage. It should be noted that more than one high voltage signal may be generated using this technique but for illustrative purposes the generation of a single voltage is described. A transformer comprises a primary winding and a secondary winding. The primary winding of the transformer is coupled through a switch to the low voltage power source. Applying a voltage to the primary winding creates a magnetic field that couples the primary winding to the secondary winding. The secondary winding is coupled to a high voltage capacitor for storing energy. Decoupling the primary winding from the voltage source creates a collapsing field that generates a current in the secondary winding that charges the high voltage capacitor. The coupling of energy from the primary winding to the secondary winding is an efficient transfer process. The secondary winding is designed to output a high voltage at low current when the field collapses. The switch coupling the primary winding is repeatedly opened and closed until the high voltage capacitor is charged to a predetermined voltage value.
Defibrillators may use two or more output capacitors to deliver energy in a programmed treatment methodology. The delivery of the energy to the cardiac tissue is coupled through a transistor. The transistor acts a switch. The voltage stored on the capacitor is typically greater than 100 volts. The transistor is enabled and disabled to control when the energy is delivered to the targeted tissue. The transistor used in this type of application is a high voltage transistor such as a power field effect transistor or an insulated gate transistor. The transistor is required to have a high breakdown voltage. For example, transistors with a breakdown between 500 and 1000 volts are used in some implantable defibrillators.
FIG. 1 is a top view of a prior art power transistor 10 coupled to a substrate 1 for use in a medical device. Substrate 1 is a printed circuit board having metal traces for coupling power transistor 10 to other components (not shown) of the medical device. Power transistor 10 has a drain, gate, and a source. Power transistor 10 has a gate contact area 40 and source contact area 50. Substrate 1 respectively has a corresponding gate contact area 20 and a source contact area 60. The drain region of power transistor 10 is the substrate of the device.
Wire bonding is used to couple the gate and source of power transistor 10 to substrate 1. Connections are made to both sides of the die of power transistor 10. As previously stated, the drain is the substrate of power transistor 10. The attachment of power transistor 10 to substrate 1 is an electrical connection of the drain to a drain contact area (not shown) on substrate 1. Electrically conductive epoxy or soldering are two common methods for electrically and physically coupling the substrate (drain) of power transistor 10 to substrate 1. Wire bonding is a mechanical process of attaching one end of a thin wire to a termination point on a side of a die and then attaching the other end of the thin wire to a pad on a substrate. Wire bond 70 couples the gate contact area 40 to the gate contact area 20. Similarly, wire bonds 60 couples the source contact area 50 to source contact area 30. Wire bonds 60 comprise multiple wire bonds to reduce the resistance of the connection. Wire bonds are problematic for medical devices. Wire bonds are fragile elements and the connection process itself can compromise the device which ultimately increases the risk to a patient.
FIG. 2 is a cross-sectional view of prior art substrate 1 and transistor 10 of FIG. 1. In general, transistor 10 is not exposed to the environment after being coupled to substrate 1. An encapsulation material 80 covers and protects power transistor 10. The amount of encapsulation material 80 used is sufficient to cover transistor 10, wire bonds 60, and wire bond 70. A secondary factor is that encapsulation material 80 must cover a vertical height of wire bonds 60 and 70 which is a substantial distance above the upper surface of power transistor 10. As shown, encapsulation material 80 extends beyond the wire bonds 60 and 70. It is highly beneficial to make implantable medical devices as small as possible to reduce invasiveness and to allow the medical device to be located proximal to the targeted tissue. Transistor 10 mounted and packaged on substrate 1 utilizes a significant amount of area as defined by encapsulation material 80. In many medical applications more than one transistor 10 is used further exacerbating the problem.
Accordingly, it is desirable to provide a smaller medical device. In addition, it is desirable to provide a high voltage power transistor capable of standing off voltages exceeding 100 volts that can reduce the footprint when physically and electrically connected to a system substrate. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.